AbsolutePath

Take an absolute path and a string representing a (possibly relative)
path and combine them into an absolute path. If the second argument is
already absolute, then the first argument gets ignored. This function also
takes care that the result is converted to Posix convention and
normalized. Also, parent directories ("..") at the front of the string
argument get canceled out against trailing directory parts of the
absolute path argument.

Regarding the last point, someone more familiar with how these functions
are used should verify that this is indeed necessary or at least useful.